Fiber optic temperature sensor

ABSTRACT

A fiber optic temperature sensor uses a light source which transmits light through an optical fiber to a sensor head at the opposite end of the optical fiber from the light source. The sensor head has a housing coupled to the end of the optical fiber. A metallic reflective surface is coupled to the housing adjacent the end of the optical fiber to form a gap having a predetermined length between the reflective surface and the optical fiber. A detection system is also coupled to the optical fiber which determines the temperature at the sensor head from an interference pattern of light which is reflected from the reflective surface.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

A portion of the work described herein was supported by the NationalAeronautics and Space Administration (NASA) under contract NAS3-27202.

RELATED APPLICATION

The application is a division of Ser. No. 08/791,025 filed Jan. 27, 1997now U.S. Pat. No. 5,870,511 and related to copending provisionalapplication No. 60/010,756 filed Jan. 29, 1996.

BACKGROUND OF THE INVENTION

The present invention relates generally to a temperature sensor and,more specifically, to a fiber optic temperature sensor capable ofmeasuring a wide range of temperatures.

In various applications such as supersonic or hypersonic aircraft, it isdesirable to measure temperatures over a large temperatures range usinga single sensor. The desired temperature range for such applications mayreach as low as -50° C. and extend up to about 1,000° C.

Conventional temperature measuring devices such as thermistors,thermocouples and bi-metal type devices are undesirable for use inaircraft applications. Such devices are vulnerable to electromagneticinterference, are heavy and may cause sparking.

Sensors employing optical fibers have been used for variousapplications. Fiber optic sensors are lighter in weight thanconventional sensors, are not susceptible to electromagneticinterference, possess larger band widths and have increased safety dueto being less susceptible to sparking. Known fiber optic sensors includepyrometric sensors which measure the radiant energy from a body.Pyrometric sensors are particularly suited for relatively hightemperatures. Florescent decay sensors are another type of fiber opticsensors. One problem with these conventional fiber optic sensors is thatthey possess inadequate dynamic range, lack measurement stability andhave an unacceptably short lifetime.

One problem common to all temperature sensing devices is that complexcalibration procedures are required when the devices are replaced. Suchcalibration procedures require a significant amount of time to implementin the aircraft industry. In particular, an easy or no calibrationprocedure is highly desirable so that a sensor may easily be removed andreplaced while requiring a minimum amount of aircraft down time.

It is therefore desirable to provide a temperature sensor which has alarge temperature range is immune from electromagnetic interference, islightweight, accurate and long-lived.

SUMMARY OF THE INVENTION

One feature of the present invention is a temperature sensor whichutilizes a light source which transmits light through an optical fiberto a sensor head received at the opposite end of the optical fiber fromthe light source. The sensor head has a sensor housing coupled to theend of the optical fiber. A metallic reflective surface is coupled tothe housing adjacent the end of the optical fiber to form a gap having apredetermined length between the reflective surface and the opticalfiber. A detection system is also coupled to the optical fiber whichdetermines the temperature at the sensor head from an interferencepattern of light which is reflected from the reflective surface.

One feature of the present invention employs a two portion reflectivesurface. The first portion of the reflective surface is made of a firstmetal and the second portion of the reflective surface is made of asecond metal. The metals preferably have a different coefficient ofthermal expansion so that an interference pattern is reflected into theend of the optical fiber.

In another feature of the present invention, the interference pattern oflight is generated from the combination of light reflecting from ahomogenous reflective material and reflecting from the end of theoptical fiber.

In yet another feature of the invention, the light detector system maybe housed on a computer board. The light detector system may be formedof a plurality of charge coupled devices so that the interferencepattern may be measured and a temperature determined therefrom.

In yet another feature of the invention a connector may be used toconnect the detection system with the sensor head, that is, one-half ofthe connector and optical fiber having the desired length and the sensorhead may be an individual unit which may be calibrated separately in aremote location than at the point of installation. The connectorassociated with the sensor head may contain a memory chip which storesthe calibration data therein when the connector is connected to thedetection system, the memory chip is read so that the temperature may bedetermined. The memory chip may, for example, contain a lookup tablecontaining data for fringes at various temperatures for example, at aspacing of 5°. To determine a temperature the fringe profile is measuredfrom the sensor and is subtracted from each of the profiles in thelookup table. The closest profile in the lookup table is determined bysubtracting each of the profiles in the lookup table from the datameasured from the sensor. When the data are subtracted, the nearest zerois determined to be the temperature. If the temperature is between twomeasurements, an interpolation may be performed to more exactlydetermine that temperature.

One advantage of the present invention is that the calibration can beperformed in a controlled environment prior to operation of the sensor.An old sensor can be removed and the new sensor placed into a system.The calibration data will then be read by the system to performtemperature calculations.

Another advantage of the present invention is that if the sensor passesthrough a reference temperature, the data stored in the computer. Theupdate process compensates for any aging effects one may experience.

In yet another feature of the invention, a method for manufacturing atemperature sensor head comprises affixing a reflector in a housing andplacing a hollow tube around the optical fiber. The method also includescoupling the first end of the optical fiber to the housing apredetermined distance from the reflector. The method for assemblyfurther includes coupling the tube to the housing and coupling a fittingaround the optical fiber. The method also includes coupling the fittingto the hollow tube.

In one aspect of the method of assembly the sensor head assembly may beinserted into a protective sheath to enhance vibration resistance andthermal conductance. The space between the sheath and the sensor headmay be filled with a thermally conductive and vibration damping powder.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent from the following detailed description which should be read inconjunction with the drawings in which:

FIG. 1 is a diagrammatic view of a fiber optic temperature measuringsystem according to the present invention;

FIG. 2 is a cross-sectional view of a sensor head according to thepresent invention;

FIG. 3 is a cross-sectional view of an alternative embodiment of asensor head;

FIG. 4 is a connection method for connecting a sensor head to an opticalfiber connector;

FIG. 5 is another alternative cross-sectional view of a sensor head;

FIG. 6 is a cross-sectional view of another embodiment of a sensor headin a protective sheath;

FIG. 7 is a cross-sectional view of a connector used to connect thesensor unit to a detection system;

FIG. 8 is a plot of a normalized spectral output versus wave length at afirst temperature;

FIG. 9 is a plot of normalized spectral output versus wave length at ahigher temperature than that of FIG. 8;

FIG. 10 is a plot of a referenced spectrum, actual temperature spectrumand calculated fringes which are used to calculate temperature; and

FIG. 11 is a sum of differences of fringe data used to calculatetemperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings like reference numerals are used toidentify identical components in the various views. Although theinvention will be illustrated in the context of a fiber optic sensorhaving a large temperature range, it will be appreciated this inventionmay be used with other applications requiring less temperature range.

Referring now to FIG. 1, a temperature sensing system 10 has a sensorunit 12, a light transmitting and receiving unit 14. Sensor unit 12extends to the location in which the temperature is to be measured.Sensor unit 12 provides a light interference pattern to lighttransmitting and sensing unit 14. Light transmitting and receiving unit14 converts the interference pattern into a temperature reading.

Sensor unit 12 generally comprises a sensor head 16, a first opticalfiber 18, a second optical fiber 20, a sensor head connector 22 and asensor unit connector 24. Sensor head 16 is located at the positionwhere the temperature is to be determined. Sensor head 16 may, forexample, be placed in the exhaust gas stream of a jet engine. Firstoptical fiber 18 is connected between sensor head connector 22 andsensor head 16. First optical fiber is preferably formed of a hightemperature resistant optical fiber such as sapphire. Sapphire is usedonly for the very end of the sensor which will be subject to hightemperatures since sapphire has low transmissivity and is relativelyinflexible. The refractive index of sapphire is about 1.77. Secondoptical fiber 20 is preferably a silica based optical fiber. Secondoptical fiber connects sensor head connector 22 to sensor unit connector24. Silica based optical fiber is more flexible and cheaper thansapphire based optical fiber. Silica fiber also has a somewhat lowerrefractive index of about 1.48. Consequently, it is preferred that themajority of the distance between sensor head 16 and sensor unitconnector 24 is made from silica based optical fiber. For simplicity,using a single optical fiber and eliminating sensor head connector 22may be desirable.

Sensor head connector 22 is preferably formed of a standardbutt-coupling optical fiber connector. One example of a suitableconnector is an SMA connector, which is common in the industry. Sensorhead connector 22 butt-couples first optical fiber 18 to second opticalfiber 20.

Light transmitting and receiving unit 14 has a mating half of sensorunit connector 24, an optical coupler 26, a light digitizer 28, a lightsource 30, an optical fiber 32 and an optical fiber 34. Optical fiber 34is used to connect optical coupler 26 to light source 30.

Optical coupler 26 is used to couple light generated from light source30 which is to be transmitted to sensor head 16 through optical fibers18 and 20. Optical coupler 26 is also used as a beam splitter to sendthe light modulated by sensor head 16 to light digitizer 28.

Light digitizer 28 may for example be a spectrometer which divides thelight up into its wave length components. Light digitizer 28 may use alinear detector such as a series of charge coupled devices (CCD). Lightdigitizer 28 converts the detected light signal from sensor 16 into adesirable output format.

Light source 30 is preferably a wide band light source such as a whitelight source. One example of a desirable white light source is atungsten-halogen source.

Light transmitting and receiving unit 14 may also have a centralprocessing unit (CPU) 36 associated therewith. CPU 36 is used to performmathematical calculations further described below. With the digitizedoutput of light digitizer 28 a display 38 may be used to display thetemperature as calculated by CPU 36 of the sensor head 16. Lightdigitizer 28 and optical coupler 26 may be contained on a computer boardwhich is inserted into CPU 36. Such a light digitizer is manufactured byOcean Optics. It is also preferred that light source 30 is contained onsuch a computer board. However, a standardized board contained aspectrometer and light source was not known at the time of thisapplication.

Referring now to FIG. 2, one embodiment of a sensor head 16 is shown.Sensor head 16 has a housing 40 which holds a reflector 42 having areflective surface 44 a predetermined distance d away from an end 46 ofoptical fiber 18. Optical fiber 18 is held to housing 40 with a hightemperature adhesive 48. In this embodiment light traveling from thelight source towards sensor head is reflected by two surfaces andcombined to form an interference pattern. Light is reflected at the endsurface 46 of optical fiber 18 due to the air-fiber interface. It hasbeen experimentally determined that approximately 4% of the light poweris reflected back into the optical fiber 18 from the end surface 46. Theremaining light travels out of the end of optical fiber 46 and reflectsfrom reflector surface 44 and re-enters the optical fiber 18. It hasbeen experimentally determined that about 90% of the reflected lightre-enters the fiber at end surface 46. The combination of lightreflecting from end surface 46 and the light reflecting from reflectivesurface 44 will generate an interference fringe pattern. Theinterference fringe pattern is a combination of the reflected lightwhich is superimposed vectorially. The distance d between end surface 46and reflective surface 44 increases as the temperature increases. Thisis mainly due to the differences of the coefficients of thermalexpansion of the optical fiber 18 and the sensor housing 40. Thechanging distance d causes the interference pattern to vary as afunction of temperature.

Reflector 42 is made of a metallic material so that nearly 100% of thelight that reaches the reflector surface 44 is reflected. It is alsopreferred that the reflector 42 is preferably made of an oxide resistantmaterial so that an oxide does not form on reflective surface 44. If anoxide forms on reflector surface 44 the distance d may be changed andthus a potential error may occur in the measurement. Materials whichhave been used to form reflector surface 44 include ZGS (a Pt and 10% Rhalloy) and platinum.

Sensor housing 40 is also preferably made of metallic material. Sensorhousing 40 may for example be made of ZGS or platinum.

Optical fiber 18 is shown having a cladding 50. It may also be removedin the portion near sensor head 16. Adhesive 48 bonds sensor housing 40to the optical fiber 18. Adhesive 48 must be capable of withstanding thetemperatures that sensor head 16 may be subject to. Adhesive 48 may, forexample, be a high temperature cement or a ceramic adhesive. One exampleof an acceptable ceramic adhesive is made by Cotronics and is calledRESPOND CERAMIC ADHESIVE 904HP.

Sensor housing 40 may have a U-shaped groove 52 which circumscribes thehousing adjacent to the distance d between end surface 46 and reflectivesurface 44. U-shaped groove 52 may be used to bend or manipulate thehousing and thus bend the orientation of the first optical fiber 18 withrespect to reflective surface 44. A slight bend at U-shaped groove 52may be used to permanently set the maximum visibility.

Reflective surface 46 is preferably planar and smooth. It is preferredthat reflective surface be polished for example by a conventionalpolishing process to obtain a smooth interface. Polishing may beaccomplished by using a diamond paste.

Referring now to FIG. 3, an alternative sensor head 16 is shown. In thisembodiment the optical fiber 18 is not secured to housing 40 using anadhesive. Housing 40 has an integrally formed receptacle portion 54which is used to hold a shoulder 56 formed in optical fiber 18. Onemethod for forming shoulder 56 and optical fiber 18 is to etch the endof optical fiber 18. Etching may be accomplished by dipping the end 46into a potassium hydroxide for a pre-determined amount of time so thatdiameter D₁ is reduced to diameter D₂. To stop the etching process astop-off material may be applied to optical fiber 18 when diameter D₂ isreached.

Referring now to both FIGS. 3 and 4, sensor head 16 is coupled to anouter tube 58. The opposite end of outer tube 58 is connected to aconnector 60. Outer tube 58 is preferably formed of a material stable inthe heat experienced by sensor head 16. Optical fiber 18 extends betweenfiber connector 60 and sensor head 16 within outer tube 58. Opticalfiber 18 is longer than the distance between connector 60 and sensorhead 16. Optical fiber 18 is squeezed into outer tube 58 so that thebending of the optical fiber 18 provides a spring force to the shoulder56 of optical fiber. This spring force will hold shoulder 56 againstreceptacle 54 without the necessity of a bonder. Both connector 60 andsensor head 16 may be bonded in an appropriate manner to outer tube 58.

Referring now to FIG. 5, another alternative embodiment of sensor head16 is shown. In this embodiment sensor head 16 has outer tube 62.Surface 46 of optical fiber 18 is held by holder tube 64 apre-determined distance from reflective surface 66 of reflector 68.Reflective surface 66 is preferably formed of a first portion 70 and asecond portion 72. Each portion has a different coefficient of thermalexpansion. First portion 70 may be formed of stainless steel. Secondportion 72 may be formed of Inconel 601. First portion 70 and secondportion 72 may be a pair of half round rods. First portion 70 and secondportion 72 may be welded together at welds 73, for example, at one endof outer tube 62.

In this embodiment, the optical fiber 18 transmits a light to bereflected from both the first portion 70 and second portion 72 ofreflective surface 66. As the temperature of sensor head 16 increases,the distance D₃ becomes different than distance D₄. The difference indistances will cause light emitted by optical fiber 18, once reflectedby first portion 70 and second portion 72, to form an interferencefringe pattern. The corresponding change in the distances D₃ and D₄corresponds to the temperature of the sensor head 16.

Referring now to FIG. 6, a sheath 74 may be used to protect sensor head16 from damage. A tip portion 75 of the sheath 74 may be shaved thin toincrease heat conduction. As shown, sensor head 16, similar to that ofFIG. 2, is shown. Like numerals from that of FIG. 2 will be used tonumber like components in FIG. 6. Sensor head configurations such asthat shown in FIGS. 3, 4 and 5 may also be utilized in thisconfiguration. A first tube 76 is coupled to housing 40 with cement 85.First tube 76 may, for example, be made of a metallic materials such asplatinum. A second tube 78 which is preferably made of stainless steelis used to connect first tube 76 to a ferrule 80. In this embodiment, itis preferred that housing 40 and first tube 76 are made of the samematerial for example platinum. First tube 76 may be spot welded tohousing 40. First tube 76 is inserted into the second tube 78 so thatany coefficient of thermal expansion mismatch causes no harm. Theferrule 80 is then connected to second tube 78 by spot welding. Theferrule 80 may be connected to sapphire fiber with cement 81. Prior toassembly, if required, a slight bend may be made in housing 40 atU-shaped grooves 52 to obtain maximum fringe visibility. Sheath 74 maythen be slid onto ferrule 80.

The cavity 82 between second tube 78 and sheath 74 may be filled with apowder 84 to absorb shock and promote thermal transfer to housing 40.Powder 84 may be made from a material such as BN or MgO. Once cavity 82is filled with powder 84, collar 83 may be bonded to sheath 74 andferrule 80. The collar 83 may support SMA connector. Second tube 78helps minimize thermal transport to ferule 80 and also acts as a heatsink for heat transported down the sapphire fiber.

Referring now to FIG. 7, connector 24 is shown in more detail. Connector24 preferably has a male portion 86 and a female portion 88. Maleportion 86 is used to connect optical fiber 20 eventually to sensor head16. Female portion 88 may be a connector mounted on a computer board.Male portion 86 may have threads 90 which are used to couple to threads92 on female portion 88. It is preferred that both sets of threads 90and 92 are formed of a metallic material. Male portion 86 may have anoptical fiber holder 94 which is connected to threads 90. Female portion88 may also have a holder 96 to hold optical fiber 20.

In day to day use, male portion 86 will be associated with a singlesensor head and its associated optical fiber. Male portion 86 can beremoved to change sensor head 16. To change sensor head 16 a new maleportion, optical fiber and sensor head are all replaced.

Male portion 86 contains a memory chip 98. Memory chip 98 is used tostore calibration data for the particular sensor head as will further bedescribed below. Memory chip 98 is coupled through an electrode 100 inmale portion 86. When male portion 86 is connected to female portion 88it is preferably connected to an electrode 102 and female portion 88.When male portion 86 is connected to female portion 88, the informationcontained in memory chip 98 is used by the CPU to calculate thetemperature based on the interference fringe pattern reflected fromsensor head 16. Threads 90 and threads 92 are preferably formed ametallic material so that the metal may act as a ground for memory chip98. Memory chip 98 may be a read only type memory; however, memory chipmay also be a RAM type memory so that the memory may be updated. Forexample, calibration data stored in memory chip 98 may be renewed eachtime the sensor head passes through a reference temperature. This wouldcompensate for any deteriorations in the fiber and in the sensor head.

An alternate method to achieve automatic calibration is to replaceconnector 22 in FIG. 1 with a connector such as that shown in FIG. 7. Inthis case, optical fiber 20 will require an additional electrical wireand ground connection. With this configuration, optical fiber 20 doesnot have to be replaced.

Referring now to FIG. 8, a normalized spectral output is plotted verseswave length in nanometers in the solid line. The interference fringepattern is normalized by a spectrum obtained at a reference temperature,for example, of 20° C. The dotted line is the average fluctuation. Bymeasuring the spacing between the fringes, that is, the done distancebetween λ_(i) and λ_(i+1), the distance between the end of the fiber andthe reflective surface may be determined. The relationship between thedistance D and the wavelength λ_(i) and λ_(i+1) is expressedmathematically as d=2(1/λ_(i) -/λ_(i+1)) . For a given set of data, thedistance d may be redundantly determined to minimize measurement error.From each fringe the value of the distance between the surface of thereflector and the surface of the optical fiber can be redundantlycalculated. Once the value for the distance d is obtained, the distanced can be converted into temperature by multiplying the distance d by aconversion coefficient which may be experimentally determined. By usingsuch a method, the reflectivity of the reflective surface only causes anamplitude change in the fringe pattern. The fringe width is notinfluenced by the reflectivity change, therefore the measurement is notaffected. If the deterioration of the surface becomes so severe that aportion of the fringe losses its visibility, that portion of the fringemay be excluded from the calculation.

The fringe pattern (a digital signal) of FIG. 8 is obtained throughlight digitizer 28. The CPU may then utilize the data for mathematicalmanipulation and then determine the temperature of the sensor head. TheCPU may retrieve the calibration coefficients from the memory chip 98.

Referring now to FIG. 9, the interference fringe pattern similar to thatof FIG. 8 is shown, except at a higher temperature. As the temperatureincreases, the frequency increases. The desirability of white light isillustrated here since white light puts out a wide spectrum of light.The wider the spectrum of light, the greater the number of interferencefringe patterns that are used in the calculations.

Referring now to FIG. 10, a reference spectrum pattern R and an actualspectrum pattern A at a particular temperature is plotted with respectto wave length. Also plotted in FIG. 10 is a calculated fringe pattern Nwhich is a normalized fringe pattern. The normalized fringe pattern iscalculated by subtracting the reference fringe pattern from the actualdata fringe pattern and dividing by the average intensity of thereference pattern.

The calibration data stored in memory chip 98 may contain a plurality ofcalibration fringe patterns. These calibration fringe patterns may, forexample, be taken at regular intervals. The calibration fringe patterns,for example, may be taken at every 0.01° C. However, in many situationsmemory size is limited. A more practical approach would be to takecalibration fringe patterns at approximately every 5° C. Each fringepattern may have somewhere in the neighborhood of 900 spectral points.The calibration fringe patterns are stored in the memory chip 98 andused by the CPU for its calculation.

The normalized fringe pattern is first obtained for an unknowntemperature. The absolute distance of the measured pattern from each ofthe calibration fringe patterns is calculated for every wave length. Theabsolute distance values are then summed up.

Referring now to FIG. 11, a plot of the sums of the absolute differencevalues is plotted against temperature measured. The absolute distancefor the closest calibration pattern would be zero if the calibrationpattern matched exactly. It is most likely that the absolute distancewill be close to zero but not exactly zero, since data was taken only atevery 5° C. An interpolation may be performed to estimate the actualtemperature within the 5° range.

When using a 5° interval, the present example measured somewhere between445° C. and 450° C. Using algebra, the actual temperatures thenestimated to be the minimum temperature point of a parabolicapproximation. Using the accuracy of such a method was determined withinplus or minus 0.3°.

The above methods for calculating a temperature based on the mount of alight reflected from a sensor head may only be performed up to apredetermined temperature since the metallic material that the sensorhead is made from may start to glow like a black body. The glowing lightcontributes to an increase in background noise and determining thetemperature range with the background noise becomes overwhelming. Thelight source may then be switched off and the light digitizer 28 may beused as a pyrometer. The light digitizer can determine the radiantenergy (i.e., wave length of light) of the sensor head. This informationmay be compared to information contained in the memory chip duringcalibration. When the temperature goes below the predeterminedtemperature, the light source may then be switched on and theinterference fringe patterns are used to calculate a temperature asdescribed above.

While the best mode for carrying out the present invention has beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims:

What is claimed is:
 1. A method for measuring temperaturecomprising:transmitting light in an optical fiber to a sensor head;reflecting a first light from a first surface in said sensor head;reflecting a second light from a second surface in said sensor head;generating an interference pattern by combining said first light andsaid second light; refracting said interference pattern intowavelengths; retrieving calibration data from a memory, said calibrationdata comprising a plurality of reference patterns corresponding to aplurality of previously measured reference temperatures; and determiningthe temperature by comparing said interference pattern with thecalibration data.
 2. A method for measuring temperature as recited inclaim 1, wherein said step of reflecting a first light comprises thesub-step of reflecting said first light from a reflective surface apredetermined distance from the optical fiber.
 3. A method for measuringtemperatures as recited in claim 1, wherein the step of reflecting asecond light comprises the sub-step of reflecting said second light froman end of said optical fiber.
 4. The method for measuring temperaturerecited in claim 1 wherein said step of reflecting a first lightcomprises the substep of reflecting said first light from a firstportion of a reflective surface, said first portion made of a firstmaterial, and said step of reflecting a second light comprises thesubstep of reflecting said second light from a second portion of saidreflective surface, said second portion made of a second material havinga different coefficient of thermal expansion than said first material.5. The method for measuring temperature recited in claim 1 wherein saidstep of determining the temperature comprises the substepsof:normalizing said interference pattern; and, comparing said normalizedinterference pattern to each of said plurality of reference patterns. 6.The method of claim 1 wherein said determining step includes thesubsteps of:calculating the difference between said interference patternand each of said reference patterns at a plurality of points to obtain aplurality of absolute distance values corresponding to each referencepattern; adding said plurality of absolute distance values correspondingto each reference pattern to obtain a total distance value correspondingto each reference pattern; and, comparing each of said total distancevalues to a predetermined reference value.
 7. A method for measuringtemperature, comprising the steps of:transmitting light through anoptical fiber to a sensor head; detecting an interference patterngenerated by said sensor head; and, comparing said interference patternwith a plurality of reference patterns corresponding to a plurality oftemperatures.
 8. The method of claim 7 wherein said detecting stepincludes the substeps of:reflecting a first portion of said light off ofa first surface; reflecting a second portion of said light off of asecond surface; and, combining said first and second portions of saidlight.
 9. The method of claim 8 wherein said first surface comprises anend of said optical fiber.
 10. The method of claim 8 wherein said secondsurface is disposed a predetermined distance from said optical fiber.11. The method of claim 8 wherein said first surface comprises a firstmaterial and said second surface comprises a second material, saidsecond material having a different coefficient of thermal expansion thansaid first material.
 12. The method of claim 7 wherein said comparingstep includes the substeps of:normalizing said interference pattern;and, comparing said normalized interference pattern with each of saidplurality of reference patterns.
 13. The method of claim 7 wherein saidcomparing step includes the substeps of:calculating the differencebetween said interference pattern and each of said reference patterns ata plurality of points to obtain a plurality of absolute distance valuescorresponding to each reference pattern; adding said plurality ofabsolute distance values corresponding to each reference pattern toobtain a total distance value corresponding to each reference pattern;and, comparing each of said total distance values to a predeterminedreference value.
 14. A method for measuring temperature, comprising thesteps of:transmitting light through an optical fiber to a sensor head;detecting an interference pattern generated by said sensor head;normalizing said interference pattern relative to a reference pattern;and, measuring a distance between first and second fringes of saidnormalized interference pattern, said distance indicative of saidtemperature.
 15. The method of claim 14 wherein said detecting stepincludes the substeps of:reflecting a first portion of said light off ofa first surface; reflecting a second portion of said light off of asecond surface; and, combining said first and second portions of saidlight.
 16. The method of claim 15 wherein said first surface comprisesan end of said optical fiber.
 17. The method of claim 15 wherein saidsecond surface is disposed a predetermined distance from said opticalfiber.
 18. The method of claim 15 wherein said first surface comprises afirst material and said second surface comprises a second material, saidsecond material having a different coefficient of thermal expansion thansaid first material.
 19. The method of claim 14 further comprising thestep of multiplying said distance by a predetermined conversioncoefficient.
 20. The method of claim 14, further comprising the stepsof:measuring a second distance between said second fringe and a thirdfringe of said normalized interference pattern; and, averaging saidfirst and second distances to obtain an average distance, said averagedistance indicative of said temperature.